If you want to be an efficient language learner, you should buy an English-English dictionary to get used to reading definitions (this way, you'll also be learning how to define words!). You should always try to find out the meaning of a word considering its context, then looking it up in an English-English dictionary and as a last resort, looking it up in a bilingual dictionary. (For more, read this email on Dictionaries.)
Free On-Line Dictionaries
(recommended) Merriam-Webster On-Line 
(US American). With audios on how to pronounce the words (forget about the phonemic transcription because it's the US system and we don't use it). It's got: an English-English dictionary (by default) . A Spanish/English dictionary. A thesaurus (for synonyms and antonyms). There are other resources you can profit from on this amazing website.
Cambridge Dictionaries and corresponding Levels
Learn about Corpuses: the Cambridge Corpus
The Urban Dictionary slang and daily words people use today
Cambridge University Press free on-line dictionaries with the phonemic transcription we use but no audio.
Word Reference Dictionaries
The American Heritage Dictionary of the English Language (fourth ed. 2000) With audios and English-English definitions.
Roget's II: The New Thesaurus (third ed. 1995) - the category index (índice is table of contents; index means índice analítico, so don't mix them up!) A dictionary to look for synonyms and antonyms.
Hi teacher! Have a look at this website. Very useful!! Teresa Vaq (Y5C, 2006-07) Visual Thesaurus
Mujer Palabra/Talking People Glossaries
(experimental stage - anybody want to work on this?)
Word Spy neologisms, abbreviations
The Harry Potter Lexicon * Pronunciation of the words in the book
Difficult words you can't find anywhere!
The Hacker's Dictionary of Computer Jargon (1992)
See US/UK Spelling under Language - World Englishes - US/UK Englishes
